Because of the introduction of microwave ovens which are employed particularly for heating foods, there has been a need for receptacles which can be placed in such ovens and will support articles to be heated while allowing microwaves to pass through the receptacle without heating, charring or melting the receptacle. One type of receptacle which has been employed satisfactorily for this purpose is a tray molded from a Bulk Molding Composition (BMC) consisting of various resins reinforced with fiberglass. Conventional polyester resis systems which have been employed to make trays for this purpose have given rather poor performance in microwave applications.